In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High levels of condensed moisture inside a cable sheath system may have a detrimental effect on the transmission characteristics of a metallic conductor cable.
Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, rodent attacks or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to enter, and, if not controlled, to move longitudinally along the cable into splice closures, for example.
Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems especially in freezing environments and should be prevented.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, a metallic shield which often times is used to protect a metallic conductor cable against lightning and rodent attacks is provided with a sealed longitudinal seam. Generally, metallic shields are not preferred for use in optical fiber cables. Forming of the shields about a cable core requires the use of relatively low manufacturing line speeds. Also the use of a metallic shield is destructive of the otherwise all-dielectric property of an optical fiber cable.
Because lightning strikes may cause holes in a metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core. Water blocking materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material, in the form of a grease, causes housekeeping problems, inhibits line speeds because of the need to fill carefully interstices of the cable core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core.
Presently, many commercially available cables also include a water swellable tape. The tape is used to prevent the travel of water through the sheath system as well as its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water swellable powder which is trapped between two cellulosic tissues. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware.
The problem of cable size caused by bulky tapes has been overcome. In U.S. Pat. No. 4,867,526, which issued on Sep. 19, 1989, in the name of C. J. Arroyo, a cable having water blocking provisions is disclosed. Interposed between a core and a jacket is an elongated substrate member which comprises an impregnated non-metallic, non-woven, web-like material in the form of a tape. The tape material is relatively compressible and has sufficient porosity to permit entry of sufficient impregnating material so that it provides enhanced water blocking capability. The impregnating material may comprise a film of a water swelling or so-called superabsorbent material.
In another prior art cable, a water blockable yarn is interposed between a core tube and an outer surface of a jacket of the cable's sheath system. The yarn extends linearly along the cable or may be wrapped helically about a portion of the sheath system. The yarn may be one which is composed of a superabsorbent fiber material which upon contact with water swells and inhibits the movement of water within the cable.
Although the foregoing arrangements provide excellent water blocking capabilities, they may result in a somewhat increased cable diameter, require additional manufacturing steps such as splicing or inhibit the use of faster line speeds. What is sought after is a cable having water blocking provisions which are provided by way of an existing element of the cable system, thereby avoiding an increase in cable diameter and facilitating improved manufacturing efficiencies. Smaller sizes result in more cable on a given reel and the ability to use presently available hardware associated with cable connections. Any reduction in size must be accomplished without compromising the strength of the cable which in optical fiber cable requires a separate strength member or members. Further desirable is that the cable structure which includes water blocking provisions be relatively flexible.
Also, cables for special applications may have more demanding requirements for blocking water than for cable used in commonplace applications. For example, a typical requirement for a cable is that no water flows through a one meter cable sample when the sample is subjected to a water head of one meter over one hour. In one special application, a cable to be acceptable must not allow any more than thirty-three cubic centimeters of water to move beyond one meter of cable when subjected to a water head, i.e. pressure, of seventeen meters over six hours.
Seemingly, the prior art does not include a cable in which water blocking provisions are integrated into an element or elements of an existing cable structure substantially without any increase in cable diameter. The sought-after cable must be such that it is easily manufacturered and use commercially available materials and must be capable of blocking the flow of water under relatively high pressures.